The invention relates to brake boosters of the vacuum type and more particularly to a booster of which the jump value can be adjusted.
The term "front" refers hereafter to the direction towards the left of the drawings, that is to say towards the front of the vehicle on which such a booster is mounted.
A booster of the vacuum type comprises a casing, within which is located a piston which, by means of a diaphragm, defines a front chamber connected permanently to a vacuum source and a rear chamber connected to the said vacuum source at rest, a valve means establishing a pressure difference between the two chambers during the actuation of a control rod capable, by means of a plunger, of bearing on one of the faces of a reaction disk integral with a push rod intended for actuating a master cylinder.
A play is defined, at rest, between the central part of the other face of the disk and the plunger. This play allows the plunger to move forwards immediately, in order to control the boosting means and ensure an immediate response of the brakes when the vehicle driver begins to actuate the brake pedal.
The effect of transmitting the boosting force exerted on the piston to the push rod via the reaction disk is to compress the peripheral part of the reaction disk axially in proportion to the boosting force. This compression of the peripheral part of the reaction disk results in a deformation of the central part of this disk towards the front face of the plunger. When the boosting force exceeds a particular threshold, the play initially present at rest between the reaction disk and the front face of the plunger is taken up, so that the plunger is in contact with the reaction disk and so that the latter transmits to the brake pedal a reaction force representing the braking force exerted on the brakes of the vehicle. This well-known arrangement enables the driver to meter the braking force which he exerts on the pedal as a function of the resistance which he encounters and which increases with this force.
Such a booster is described in EP-A-0,340,059 corresponding to the U.S. Pat. No. 4,984,506 issued Jan. 15, 1991.
It emerges from the foregoing explanation that the pedal reaction begins to appear only when the boosting force generated as a result of the actuation of the brake pedal exceeds a particular threshold. This threshold is called the "jump" of the booster. It is an important characteristic of the booster. In fact, although it is necessary for the jump to exist in order to obtain an immediate response of the brakes during an actuation of the pedal, the manufacturers of motor vehicles usually want the value of this jump to remain within particular limits, so that the boost does not reach too high a value without an increase in the pedal reaction.
However, especially in view of the production tolerances of the various component parts of the booster, there can be considerable differences in the value of the jump from one booster to another. Moreover, at the present time there is no quick and practical means for controlling or adjusting the value of the jump of the booster, and therefore the abovementioned requirements are not always met, unless complex checks involving a high outlay are made.